Add webhooks section to OpenAPI setup

[ethanpalm]

Documentation changes

This PR adds info on web hooks to https://mintlify.com/docs/api-playground/openapi-setup. Right now, there is just a brief mention about webhooks in a note which is too hard to find. This adds a section with info that webhooks are supported in OpenAPI 3.1+ and how to specify a webhook in a n MDX page.

Closes

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[handotdev]

I think the disclaimer on the version should be closer with where the content is at. I don't think it needs to be at the high level overview

[ruhanponnada]

lgtm